The National Simulation Resource for Stochastic Micropopulation Models provides an environment for Monte Carlo modeling of structured micropopulations. Core research in simulation methodologies enhances collaborative development of categorical models in the areas of infectious, chronic, and genetic disease as well as cognitive processing. Commonalities at the conceptual, coding and implementation levels allow easy maintenance, sharing and reuse of the developed software. Current areas of categorical research examine the epidemic spread of HIV/AIDS, influenza and an animal model of herpes virus; population interventions against heart attack, stroke and other cardiovascular diseases; familial transmission of diabetes, epilepsy, schizophrenia and lung cancer; and artificial neural networks modeling visual processing. The simulation shell can also generate structured populations with given genetic, demographic and physiological characteristics to test analytic systems. An outreach program supports external users with dissemination, training and service. The goal of the Resource is to provide an integrated simulation environment to conduct biomedical research, from the creation of a model to the analysis of simulation results, and allows investigators to incorporate experimental data and evaluate hypotheses. A simulation shell termed SUMMERS, based on a single conceptual model, contains a library of VAX/VMS FORTRAN modules allowing the simulation of a generic population of individual members in interactive groups transferring between multiple activity states at discrete intervals. Researchers use keywords to modify existing categorical models or assemble program modules from the simulation shell and its templates to develop their own applications. A separate sensitivity analysis system assesses the relative importance and plausible range for model parameters given the outcome data, and can optimize parametric values to fit pre-specified outcomes. Prototype expert systems provide control frameworks to help build, implement, modify and analyze these SUMMERS-based models. New initiatives planned include incorporating these subsystems into the SUMMERS shell, adapting the SUMMERS code to C and UNIX-based computer systems, enhancing interactive and graphics-based use, and developing object-oriented structures within SUMMERS to represent social networks and other micropopulation features.